Question

The boiling point of water is higher than liquid HF. The reason is that:

• A. Hydrogen bonds are stronger in water.
• B. Hydrogen bonds are stonger in HF.
• C. Hydrogen bonds are larger in number in HF.
• D. Hydrogen bonds are larger in number in water.

Answer 1

The Correct Option is D

• Hydrogen Bonding: Both water (H₂O) and hydrogen fluoride (HF) exhibit hydrogen bonding due to the high electronegativity of oxygen and fluorine, respectively.
• Number of Hydrogen Bonds: A water molecule can form up to four hydrogen bonds (two as a donor and two as an acceptor). Each hydrogen atom in water can form a hydrogen bond, and each oxygen atom has two lone pairs that can accept hydrogen bonds. In HF, however, each HF molecule can form only two hydrogen bonds on average (one as a donor and one as an acceptor). The fluorine atom has three lone pairs but only one hydrogen to donate.
• Boiling Point: The boiling point is related to the amount of energy needed to overcome intermolecular forces. Because water can form a greater number of hydrogen bonds per molecule than HF, more energy is required to break these bonds and vaporize water, resulting in a higher boiling point.
• Strength of Individual Hydrogen Bonds: While the H-F hydrogen bond *can* be slightly stronger than the O-H hydrogen bond in some situations, the *number* of hydrogen bonds per molecule is the dominant factor in determining the overall intermolecular force strength and, consequently, the boiling point.

Therefore, the larger number of hydrogen bonds in water, rather than the strength of individual bonds, is the primary reason for its higher boiling point compared to HF.

Correct Answer: Hydrogen bonds are larger in number in water.